Turbine bucket cover assembly



Nov. 8, 1960 A. w. RANKIN 2,959,393

TURBINE BUCKET COVER ASSEMBLY Filed April 16, 1956 Inventor 1 Andrew W. Rcmki r1 His A'Lorey United States Patent O TURBINE BUCKET COVER ASSEMBLY Andrew W. Rankin, Schenectady, N .Y., assignor to General Electric Company, a corporation of New York Filed Apr. 16, 1956, Ser. No. 578,406

1 Claim. (Cl. 253-77) This invention relates to elastic uid bucket wheels for turbines or compressors and more particularly to a bucket cover assembly for tying together and covering over the ends of the buckets to increase the vane rigidity and prevent the leakage of motive uid past the vane ends.

Adjacent turbine buckets are normally connected together by some form of cover band attached to the outer ends of the vanes to conne the operating fluid within the working passage and to increase the rigidity of the vanes. The covers are subjected to relatively great tangential forces exerted by the operating fluid against the buckets, centrifugal forces, due to the wheel rotation, acting outwardly on the cover band and forces exerted by the operating uid acting directly against the surface of the cover. These forces highly stress the cover and the attaching means between the cover and vane. The usual form of covers consisting of a single tie member riveted onto the bucket ends requires the use of high strength cover materials of substantial weight to withstand the high stress concentrations in the cover portion of the cover assembly. In addition, this usual construction requires the use of bucket materials which can be readily peened to secure the cover to the bucket.

While these constructions have been satisfactory, it is noted that in the cases where the cover is riveted it is not uncommon to require a complete new set of buckets when the covers have to be removed and replaced, for any reason, because the bucket material will not permit a second peening operation.

Accordingly, it is an object of this invention to provide a new and improved turbine bucket wheel cover assembly which can be removed and replaced as freqnently as desired without harming either the bucket or cover member.

A further object is to provide a light weight, high strength bucket cover in which the bending stresses in the span between the buckets are held to satisfactory values even for the wide spaces provided between buckets in new design turbines.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is a View partly in section taken along lines 1-1 of Fig. 2 illustrating the method of securing the cover assembly to the bucket; and

Figs. 2 and 3 are plan and side views respectively of the arrangement shown in Fig. l.

Generally stated, the invention is practiced by providing a circumferentially ribbed arcuate bucket cover which is secured to the buckets by fastening means interconnecting the cover ribs and the bucket tenons extending through openings in the cover.

Referring to Fig. 1, there is illustrated a conventional bucket 2. The bucket 2 includes a base portion which has conventional fastening means for connecting the buckets to a turbine rotor, and a vane portion 2a which is exposed to the motive uid. The details relating to ICS the bucket base portion and the fastening means therefore are not shown since they are not important to an understanding of the present invention.

The vane 2a defines two rectangular tenons 2b which extend radially outward from the thickened intermediate portion of the blade and it is to these tenons that the applicants novel cover is secured.

The cover assemblies form a circumferential band around the bucket wheel and comprise a plurality of arcuate segments 1 each having a plate portion 1a and a plurality of radially extending circumferentially disposed ribs 1b. Each arcuate segment 1 bridges the spaces between the adjacent buckets and projects axially slightly beyond the leading and trailing edges of the vane 2a as shown at 1e. While in a present design eight buckets are tied together by each segment, it is obvious that any suitable number may be included in a group. It can thus be appreciated that the cover segments 1 substantially enclose the space between the adjacent buckets to prevent the motive uid. from spilling outwardly past the bucket ends. The ribs 1b greatly strengthen the cover between the adjacent buckets to hold the span bending stresses in the cover to satisfactory values even where relatively wide spaces are provided between adjacent buckets. This construction also permits the use of a relatively low strength cover material since the ribbed construction holds the stress levels in the cover to relatively low values. Each cover segment 1 has rectanguflar openings 1c in the plate portion 1a between the ribs 1b for receiving the tenons 2b. The ribs 1b are located adjacent the openings 1c to strengthen the cover around the tenon holes where, in the case of conventional flat bucket covers, fatigue cracks frequently occur. These openings 1c are the same width as but slightly longer than the tenons 2b. This clearance space is required, as will be apparent to those skilled in the art, to permit the arcuate cover segments to be installed in a radial direction over the projecting tenons.

The cover segments 1 are connected to the buckets 2 by pins 3 located in aligned axially extending openings 1d, 2c in the ribs 1b and tenons 2b respectively. The pins 3 may be secured in position in any suitable manner such as by peening the ends thereof as shown at 3a. This location of the fastening means between the bucket and cover places the stress concentrations in the cover away from the cover tenon holes 1c and up into the relatively llightly stressed portions of the ribs 1b. Also, by utilizing a plurality of tenons, the shear stresses in the pins 3 are substantially minimized. It will be appreciated that further reduction in pin shear stress can be accomplished by providing additional circumferential ribs 1b with cooperating tenons projecting from the leading and trailing edges of the vanes 2a.

The cover is assembled on the buckets in the following manner.

Each cover segment 1 is disposed over the buckets 2 with the tenons 2b being received in the openings 1c. The axially aligned vopenings 1d, 2c are then formed by a conventional drilling operation, and the pins 3 are inserted and riveted over at both ends, as shown at 3a, to prevent them from becoming disengaged from the bucket wheel.

Thus, it can be seen that the invention provides a ribreinforced bucket cover which gives maximum strength for a minimum of weight, keeps the span bending stresses in the cover at a minimum, and can be removed and replaced as frequently as required without harming either the bucket or the cover, the dowel pins 3 being the only expendable part of the assembly.

While a single embodiment of the invention has been described in detail herein, it will be obvious to those skilled in the art that changes and substitutions of equiva` lents might be made. For example, the tenons can be carried out to the leading and trailing edges of the blades and they and the associated openings 1c need not necessarily be rectangular.`

It is, of course, desired to cover by the appended claim all such modifications4 as `fall Within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

In a turbomachine bucket wheel assembly, the combination of a plurality of circumfcrentially spaced buckets each having a base portion and a vane portion, each vane portion having at least two tenons extending radially outwardly from the tip thereof, a cover comprising a plurality of lightweight arcuate segments disposed on the vane tip portions so as to be Supported thereby, each cover segment spanning the gap between adjacent vanes and having a plate portion with a plurality of radial openings positioned to receive a plurality of the radially oute wardly extending tenons, each cover segment including also at least three radially outwardly extending circumferentially disposed ribs on the plate portion located adjacent the radial openings for reinforcing the cover segment, the ribs being disposed in juxtaposition to the radially outwardly extending tenons, the ribs and adjacent tenons having aligned axially extending openings, and dowel pin means extending through the axially aligned openings for securing each cover segment to the vane tenons.

References Cited in the file of this patent UNITED STATES PATENTS 764,451 Geisenhoner July 5, 1904 1,371,328 Schneider Mar. 15, 1921 1,457,404 Snyder June 5, 1923 1,466,324 Wilkinson Aug. 28, 1923 1,720,754 Baumann July 16, 1929 1,998,393 Junggren Apr. 16, 1935 1,998,951 Downer Apr. 23, 1935 2,000,692 Dimberg May 7, 1935 2,220,914 Robinson Nov. 12, 1940 2,278,041 Allen Mar. 31, 1942 2,350,309 Dahlstraud May 30, 1944 FOREIGN PATENTS 732,920 Great Britain June 29, 1955 823,442 France Oct. 18, 1937 

